pcm: ladspa: Fix segfault due to a wrong channel reference

pcm: ladspa: Fix segfault due to a wrong channel reference

Because of a typo in referencing the input array in
snd_pcm_ladspa_allocate_memory(), ladpsa PCM plugin may cause a
segfault at prepare when input and and output channels are different:
 #0  0x00007ffff78623ef in snd_pcm_ladspa_allocate_memory (pcm=0x626fa0, pcm=0x626fa0, pcm=0x626fa0, ladspa=0x621ad0) at pcm_ladspa.c:753
 #1  snd_pcm_ladspa_init (pcm=0x626fa0) at pcm_ladspa.c:834
 #2  0x00007ffff7842946 in snd_pcm_plugin_prepare (pcm=0x626fa0) at pcm_plugin.c:171
 #3  0x00007ffff784290f in snd_pcm_plugin_prepare (pcm=0x62c760) at pcm_plugin.c:162
 #4  0x000000000040256a in ?? ()
 #5  0x00007ffff7222ec5 in __libc_start_main (main=0x401d80,a argc=4, argv=0x7fffffffde28, init=<optimized out>, fini=<optimized out>, rtld_fini=<optimized out>, stack_end=0x7fffffffde18) at libc-start.c:287
 #6  0x0000000000402fdd in ?? ()

This patch corrects the wrong reference.

Reported-and-tested-by: Andreas Hartmann <mail@andreashartmann.eu>
Signed-off-by: Takashi Iwai <tiwai@suse.de>